Oxygen measuring apparatus and method with automatic temperature compensation

ABSTRACT

During the measurement of oxygen content of a gas with a solid electrolyte material capable of oxygen ion conductivity the electrolyte is heated to maintain a constant control temperature. If the ambient temperature changes, an offset or error is introduced into the oxygen measurement. An apparatus and method is provided to automatically compensate for any such offset. Thermocouple means are provided in series with the electrolyte circuit so that a thermal emf if generated due to any differential between ambient and control temperatures. The thermocouples are arranged to generate a net thermal emf which opposes the emf generated by the electrolyte by an amount which automatically compensates for any offset due to the temperature differential. If the ambient and control temperatures are the same, there will be no offset and no net thermal emf.

This invention relates to an apparatus and method for measuring theoxygen content of a gas.

More particularly, this invention relates to an apparatus and method formeasuring the oxygen content of a gas employing a solid electrolytematerial capable of oxygen ion conductivity, such as a stabilizedzirconia solid electrolyte cell. Although the present invention can beused to measure the oxygen content of the gas in any flowing stream, theinvention is described herein particularly for the measurement of theoxygen content of flue gas in a combustion process such as a boiler.

In the measurement of the oxygen content of a flue gas employing a solidelectrolytic cell, a heater is disposed in close proximity to or withinthe cell to maintain a constant control temperature at the cell.Accuracy of measurement requires the control temperature at the cell bethe same as the temperature of the flue gas stream under measurement,which is referred to as the ambient temperature. The measuringinstrument is calibrated to indicate oxygen content, concentration orpartial pressure in the gas under measurement assuming a constantambient temperature. If the ambient temperature deviates from the setcontrol temperature at the electrolytic cell, an offset or error isintroduced into the indicated measurement of oxygen concentration. Theapparatus and method of this invention automatically compensates for anydeviation in ambient temperature from the control temperature.

The electrochemical cell produces an electrical potential orelectromotive force (emf) which is indicative of the oxygen content inthe flue gas. If the control temperature established by the heaterwithin the cell and the ambient temperature outside the cell are thesame, there will be no cell offset due to temperature. If thetemperature within the cell and the ambient temperature are different,there will be a cell offset due to temperature, i.e. an error ordeviation from the correct cell indication of oxygen content in the fluegas. The offet arises due to electrical noise generated at the variousjunctions of dissimilar metals in the series circuit of which the sensorcell is a part. The various junctions of dissimilar metals produceelectrical thermocouple effects which introduce undesired emf's into thecircuit.

In a preferable mode, this invention utilizes a thermocouple system inthe series circuit which automatically compensates for any differencesbetween the cell temperature and the ambient temperature and whichautomatically compensates for cell offset due to temperature. Thethermocouple system of this invention comprises a series circuitincluding thermocouple pairs which are arranged in the circuit toprevent any emf effect due to temperature variation from reaching theoxygen indicating instrument. In the circuit the thermocouples in eachpair comprise similar but oppositely connected metals, whereby thethermocouples in each pair produce opposing emf's, whereby no net emffrom the pair is produced when the cell temperature and the ambienttemperature are the same. When there is a differential between the celltemperature and the ambient temperature, the opposing thermocouples inthe pairs of thermocouples produce a net thermal emf. The net thermalemf opposes the emf generated at the cell due to oxygen concentration byan amount equal to the cell offset and thereby compensates for any celloffset due to temperature.

If there is no differential between the cell temperature as etablishedby the cell heater and the ambient temperature as established by theflue gases, then the temperatures on the inside of the cell and on theouside of the cell will be the same. In this case, the similar butopposite thermocouples in the series will each generate emf's, but theemf's generated by each thermocouple of a pair will be equal andopposite and thereby cancel each other. There is no cell offset due totemperature when the cell temperature and the ambient temperature arethe same so that no net emf generated by the series of thermocouples isrequired.

On the other hand, if there is a differential between the celltemperature as established by the cell heater and the ambienttemperature as established by the flue gases, then the temperatures onthe inside of the cell and on the outside of the cell will be different.In this case, the similar but opposite thermocouples in the series willeach generate an emf but these emf's will be unequal as well as oppositeso that there will be a net emf generated by the thermocouple pairs.There is a cell offset due to temperature when the cell temperature andthe ambient temperature are different, and in the present system the netemf generated by the series of thermocouples will oppose and therebycompensate for the cell offset to an extent which prevents any emfeffect due to temperature variation from reaching the measuringinstrument.

In accordance with this invention, automatic compensation for celloffset due to a difference between oxygen cell temperature and ambienttemperature can be achieved in other ways. For example, temperaturemeasuring means can be installed to measure the ambient temperature ofthe boiler flue gases and the ambient temperature signal can be suppliedto a microcomputer which is programmed to correct the oxygen measuringinstrument for any offset due to deviation of ambient temperature fromthe temperature of the cell.

The apparatus of this invention can comprise a tube of solid electrolytematerial capable of oxygen ion conductivity, said tube having a closedend in communication with the gas under measurement and an open endexposed to atmospheric air. A first electrode is disposed on the outsidesurface of the tube and a second electrode is disposed on the insidesurface of the tube. A heater is disposed in close proximity to orwithin the tube for eeating the tube and maintaining a constant controltemperature at the tube. Oxygen indicating means is provided remote fromthe tube. First wire means extends between the first electrode and theindicating means and second wire means extends between the secondelectrode and the indicating means. Temperature measuring means is alsoconnected to the indicating means for measuring the ambient temperaturearound said tube. The apparatus includes temperature compensating meansfor automatically compensating for any offset in the indication ofoxygen content due to a difference between the ambient temperature andthe control temperature.

As indicated above, the temperature compensation means can comprise amicrocomputer disposed at the indicating instrument. Preferably, thetemperature compensating means comprises thermocouple junctions inseries with the electrolytic cell. The thermocouple junctions measurethe differential between the ambient and control temperatures andgenerate a net thermal emf which opposes the electrolytic cell emf dueto oxygen conductivity by an amount equal to any cell offset due totemperature, thereby automatically compensating for any offset arisingdue to a difference between ambient and control temperatures.

In a more particular description of the thermocouple mode of thisinvention, the tube of solid electrolyte has a closed end and an openend, the closed end is in communication with the flue gas and the openend is in communication with atmospheric air. A first electrode isdisposed on the inside surface of said tube. A second electrode isdisposed on the outside surface of the tube. A heater is disposed withinthe tube for heating said tube. Oxygen indicating means is provided forindicating the oxygen content of the flue gas. First and second wiremeans in series extend between said first electrode and the oxygenindicating means. The first and second wire means comprise dissimilarmetals, respectively, and are joined to each other to form a firstthermocouple junction at a position removed from said first electrode inan ambient temperature zone. The first wire means comprises the samemetal as said first electrode and is connected to said first electrode.The second wire means is connected to the oxygen indicating means. Thirdwire means extends between the second electrode and the oxygenindicating means. The third wire means and the second electrode comprisedissimilar metals, respectively, and are joined to each other to form asecond thermocouple junction. The third wire means comprises the samemetal as the second wire means. The first and second thermocouplejunctions generate first and second thermal emf's respectively if thefirst thermocouple junction, which is at ambient temperature, and thesecond thermocouple junction, which is at or near the cell temperature,are at different temperatures. This will occur if the ambienttemperature and the cell temperature are different. The first and secondthermal emf's oppose each other to provide a net thermal emf. The netthermal emf opposes the emf generated by the cell in response to oxygenconductivity to automatically compensate for any offset in cellmeasurement due to a differential between the ambient and controltemperatures. If the ambient and control emf's are the same, there willbe no net thermal emf and no offset.

This invention also relates to an advantageous probe apparatus forsupporting the electrolytic cell within a boiler or other vesselcontaining an oxygen containing gas. The probe comprises an outercylindrical housing having one end for extending into a stream of saidgas and another end for mounting at the wall of a vessel containing saidgas. An inner cylindrical housing is disposed inside of said outercylindrical housing to provide an annular space between said inner andouter cylindrical housings. A tube of solid electrolyte material capableof oxygen ion conductivity having a closed end and an open end issupported within said inner cylindrical housing with the open end facingthe interior of said inner housing and the closed end projectingoutwardly towards said one end of said outer housing. Sealing means isprovided between the tube and the inner cylindrical housing to seal theinterior of the inner housing from the interior of the outer housing.Closure means is provided at said another end of the apparatus forclosing said annular space from the atmosphere. Calibration port andplug means open to said annular space is provided for periodicallysupplying a calibration oxygen-containing gas to said annular space.

In terms of process, this invention relates to a method for measuringthe oxygen content of a gas comprising passing said gas to the outsideof a tube of solid electrolyte material capable of oxygen ionconductivity, said tube having a closed end in communication with saidgas and an open end, and said tube having an outside electrode and aninside electrode. The open end of the tube is exposed to atmosphericair. The tube is heated to maintain a control temperature. The oxygenion conductivity generates a cell emf and this emf signal is passed toan oxygen indicating instrument for indicating the oxygen content of thegas. The ambient temperature in the vicinity of said tube is measuredwith temperature measuring means to produce a thermal emf signal whichis passed to the oxygen indicating instrument. Based on the thermal emfsignal, the oxygen indicating means automatically compensates for anoffset, if any, in oxygen content indication due to a difference betweensaid ambient temperature and said control temperature.

Electrochemical cells for analyzing for oxygen content in a gas streamby measuring oxygen partial pressure are well known and suitable cellsfor this purpose are disclosed in U.S. Pat. Nos. 3,597,345, 3,865,707and 3,869,370, which are hereby incorporated by reference.

This invention will be more fully understood by reference to theaccompanying drawings in which

FIG. 1 shows the sensor cell and thermocouple circuit of this invention,and

FIG. 2 shows an advantageous probe body design for containing the sensorcell and thermocouple circuit.

The sensor series circuit of this invention is illustrated in FIG. 1.FIG. 1 shows five emf generators in the sensor series circuit. Thesefive emf generators are shown by letter designation in FIG. 1 and arelisted in the following table.

    ______________________________________                                                 Type of emf                                                          Position generator         Emf type                                           ______________________________________                                        A        Ni--Pt thermocouple                                                                             Thermal                                            B        Pt--ZrO.sub.2 thermocouple                                                                      Thermal                                            C        Electrochemical cell                                                                            Electrochemical                                    D        ZrO.sub.2 --Pt thermocouple                                                                     Thermal                                            E        Pt--Ni thermocouple                                                                             Thermal                                            ______________________________________                                    

Referring to the above table, it is seen that the thermocouples in pairA and E are similar but opposite and the thermocouples in pair B and Dare similar but opposite. Therefore, if the temperatures at positions A,B, C, D and E are all the same the thermal emf's generated by similarbut opposite thermocouples A and E will be equal and opposite and willcancel each other and the thermal emf's generated by similar butopposite thermocouples B and D will be equal and opposite and willcancel each other. Thereby, if the temperatures at each of the positionsare the same the net emf generated by the series circuit will be thecell emf with no offset due to temperature.

If some or all of the temperatures at positions A, B, C, D and E aredifferent from each other, there will be unequal thermal emf's generatedby the similar but opposite thermocouples. The thermal emf's generatedby similar but opposite thermocouples A and E will be unequal andopposite, resulting in a first net thermal emf. Also, the thermal emf'sgenerated by similar but opposite thermocouples B and D will be unequaland opposite, resulting in a second net thermal emf. Thereby, if thetemperatures at each of the positions are different there will be one ormore net thermal emf's generated. The sum of these net thermal emf'swill be equal to the cell offset due to temperature effects. Thethermocouples are arranged in the series circuit so that the sum of thenet thermal emf's will be not only equal to but also opposite to thecell offset emf, and will thereby cancel the cell offset emf. Thereby,the thermocouple arrangement in the electrical series circuitautomatically cancels any cell offset due to thermal effects.

Referring again to FIG. 1, probe body or housing 14 is inserted througha hole in boiler wall 16 and is fixedly mounted on wall 16 by anysuitable means so that it protrudes into boiler interior 18. The forwardend of housing 14 is enclosed by filter 20 and flame arrestor 22. Filter20 removes solid particulates from the flue gas and can comprisecarborundum ceramic. Flame arrestor 22 can comprise knitted stainlesssteel. Filter 20 and flame arrestor 22 are optional equipment.Electrochemical cell 24 is supported within cylindrical housing 14 andis capable of conducting oxygen ions to provide an electrochemical emfsignal indicative of the oxygen content of a gas. The materialscomprising cell 24 are well known. They include solid electrolytematerial capable of oxygen ion conductivity. Cell 24 can comprise solidsolutions of oxides of a tetravalent element from the group consistingof zirconium, thorium and hafnium and an oxide of a metal such ascalcuim, barium, strontium and lanthanum. A common composition comprisesa solid solution of zirconium oxide and yttrium oxide, commonly referredto as stabilized zirconia (ZrO₂).

Cell 24 can comprise stabilized zirconia electrolyte 26 coated withouter platinum electrode 28 and a separate inner platinum electrode 30.Cell 24 is ubular and comprises a closed end 25 and an open end 27.Closed end 25 comprises the outside of the cell and open end 27 providesaccess to the inside of the cell. The platinum electrodes are gaspermeable. Cell 24 is mounted on seal 32 which separates flue gascirculating zone 34 from atmospheric air-containing zone 36. Thereby,the outside of the cell is exposed to flue gases and the inside isexposed to atmospheric air. Heater unit 38 having electrical power leads40 is disposed within electrochemical cell 24 and maintains a constantcontrol temperature within the cell which can be equal to the normalambient temperature in zone 18.

The series circuit including zirconia electrolyte C extends from nickelwire 10 to nickel wire 12. Wires comprising alloys having thermoelectricproperties similar to nickel, such as constantan, can be substituted fornickel wires herein. Wires 10 and 12 can terminate at remote oxygenconcentration indicating instrument 42. Nickel wire 10 formsthermocouple junction A with platinum wire 44. Thermocouple junction Ais located outside of and away from cell 24 so that it is under theambient temperature influence of boiler interior 18. Platinum wire 44and inside platinum electrode 30 form a continuous conductor. Wirescomprising alloys having thermoelectric properties similar to platinum,such as chromel, can be substituted for platinum wires herein. Platinumelectrode 30 forms a continuous junction with zirconia electrolyte C, asindicated schematically at B. The circuit then passes throughelectrolyte C. In turn, platinum electrode 28 forms a continuousjunction with zirconia electrolyte C, as indicated schematically at D.Finally, outside platinum electrode 28 forms junction E with nickel wire12 to complete the circuit.

Flue gases flowing in boiler chamber 18 pass through filter 20 asindicated at 46 and then pass through flame arrestor 22 as indicated at48 to enter chamber 34. Filter 20 and flame arrestor 22 are bothoptional equipment. The flue gases can traverse chamber 34 as indicatedat 50 but can proceed only as far as seal 32, which serves as a barrieragainst further flow of the flue gases. Chamber 36 is thereforeprotected against inflow of flue gases but is exposed to and containsatmospheric air. Thereby, outside porous electrode 28 of cell 24 isexposed to flue gases and inside porous electrode 30 of cell 24 isexposed to atmospheric air.

FIG. 1 illustrates the five emf-generating elements of the seriescircuit. First, an emf-generating thermocouple at nickel-platinumjunction A which is under the influence of the ambient temperature inboiler 18. Secondly, an emf-generating thermocouple at platinum-zirconiajunction B, which is under the influence of the outer skin temperatureof cell 24. Third, the electrochemical emf generated by zirconiaelectrolyte C, which is determined by the oxygen concentration of fluegas 46. Fourth, an emf-generating thermocouple at zirconia-platinumjunction D, which is under the influenceof heater 28. Fifth, anemf-generating thermocouple at platinum-nickel junction E which is underthe influence of basically the outer skin temperature of cell 24.

It is seen that the four thermocouples A, B, D and E comprise two pairsof similar but opposite thermocouples. Thermocouple A is a Ni-Ptthermocouple while thermocouple E is a Pt-Ni thermocouple. Therefore,thermocouples A and E produce opposing emf's. Thermocouple B is aplatinum-zirconia thermocouple while thermocouple D is azirconia-platinum thermocouple. Therefore, thermocouples B and D produceopposing emf's. If all the thermocouples are at the same temperature,they produce no net thermocouple emf and the only emf produced by theseries circuit is the electrochemical emf produced by the zirconia cell.If the thermocouples are not at the same temperature, there is a netthermocouple emf which automatically cancels any thermal offset in theelectrochemical generated emf.

FIG. 2 shows an advantageous housing arrangement for electrolytic cell24 which provides ease of servicing and calibrating. Because the housingarrangement of FIG. 1 utilizes seal 32 extending between cell 24 andhousing 14 to secure the cell to the housing, the entire housing must beremoved whenever it is desired to remove cell 24 for servicing.Furthermore, seal 32 prevents a calibration gas from reaching theexterior surface of cell 24 for calibration purposes. The embodiment ofFIG. 2 avoids these difficulties by utilizing a seal arrangement whichprovides an uninterrupted annular space between the cell and the outerhousing so that calibration gas can reach the exterior of the cell andso that the cell can be easily removed from the housing for servicing.

Referring to FIG. 2, exterior cylindrical housing 60 extends into boilerspace 62 through mounting flange 64 which can be attached to a boilerwall at an opening, not shown. Housing 60 has an extension on the endextending furthest into boiler space 62 comprising coupling 66 andhousing 60 has an extension into the atmosphere comprising coupling 68.Coupling 66 supports filter 70 by means of lock screws 72 and alsosupports flame arrestor 74. Coupling 68 is provided with a calibrationport closed by plug 76 and supports electrical terminal case 78.

Internal cylindrical housing 80 is secured to flange 82 on one end.Flange 82 is mounted onto coupling 68 by means of bolts 84 and sealinggasket 86 provided between flange 82 and coupling 68. The opposite endof internal housing 80 is provided with collar 88 which supports cellseal 90 which in turn supports electrolytic cell 92. Conduit 94 extendsfrom cell 92 for carrying thermocouple and heater electrical wires 96from cell 92 to terminal case 78.

The embodiment of FIG. 2 provides continuous and uninterrupted annularspace 98 between exterior housing 60 and interior housing 80. It alsoprovides continuous and uninterrupted annular space 100 between interiorhousing 80 and conduit 94. Flue gases indicated at 102 flow inwardlythrough filter 70 and through flame arrestor 74 to the exterior of cell92. The flue gases have access to annulus 98 but are prevented fromescape to the atmosphere by means of flange 82 and gasket 86. Theatmosphere has access to the interior of cell 92 through end opening 104in conduit 94.

An advantage of the structure of FIG. 2 is that electrolytic cell 92 canbe easily removed for servicing merely by unscrewing bolts 84 and movingflange 82 horizontally outwardly, as shown. Inner housing 80 and cell 92are integral with flange 82 and are removed therewith. Another advantageof the structure of FIG. 2 is that cell 92 can be calibrated in placewhen the boiler is not operating. Calibration occurs by removing plug 76and introducing a gas of known oxygen concentration into the calibrationgas port. The calibration gas travels along annular space 98 to theexterior of cell 92.

I claim:
 1. An apparatus for measuring the oxygen content of a gascomprising;a tube of solid electrolyte material capable of oxygen ionconductivity, said tube having a closed end in communication with saidgas and an open end in communication with atmospheric air, a firstelectrode disposed on the inside surface of said tube, a secondelectrode disposed on the outside surface of said tube; heater meansdisposed in close proximity to said tube for heating said tube formaintaining a control temperature at said tube; oxygen indicating meansfor indicating the oxygen content of said gas; first wire meansextending between said first electrode and said indicating means; secondwire means extending between said second electrode and said indicatingmeans; temperature measuring means connected to said indicating meansfor measuring the ambient temperature around said tube; and ambienttemperature compensating means in said apparatus for automaticallycompensating for any offset in said measurement of oxygen content basedon a difference between said ambient temperature and said controltemperature.
 2. The apparatus of claim 1 wherein said heater means isdisposed within said tube.
 3. The apparatus of claim 1 wherein saidfirst electrode and said second electrode are platinum electrodes. 4.The apparatus of claim 1 wherein said tube comprises stabilizedzirconia.
 5. The apparatus of claim 1 wherein said first wire means andsaid second wire means comprise a material selected from the groupconsisting of nickel and constantan.
 6. The apparatus of claim 1 whereinsaid ambient temperature compensating means is in said oxygen indicatingmeans.
 7. The apparatus of claim 1 wherein said ambient temperaturecompensating means includes temperature measuring means.
 8. Theapparatus of claim 1 wherein said ambient temperature compensating meansincludes a thermocouple junction in said first wire means.
 9. Anapparatus for measuring the oxygen content of a gas comprising;a tube ofsolid electrolyte material capable of oxygen ion conductivity, said tubehaving a closed end in communication with said gas and an open end incommunication with atmospheric air, a first electrode disposed on theinside surface of said tube, a second electrode disposed on the outsidesurface of said tube; a heater in close proximity to said tube forheating said tube; indicating means for indicating the oxygen content ofsaid gas; first and second electrical wire means in series extendingbetween said first electrode and said indicating means; said first andsecond wire means comprising dissimilar metals, respectively, and joinedto each other at a position removed from said first electrode to form afirst thermocouple junction; said first wire means comprising the samemetal as said first electrode and connected to said first electrode;said second wire means connected to said indicating means; third wiremeans extending between said second electrode and said indicating means;said third wire means and said second electrode comprising dissimilarmetals, respectively, and joined to each other to form a secondthermocouple junction; said third wire means comprising the same metalas said second wire means; said first and said second thermocouplejunctions generating first and second emf's respectively; and said firstand second emf's opposing each other.
 10. The apparatus of claim 9wherein said first electrode and said second electrode compriseplatinum.
 11. The apparatus of claim 9 wherein said first wire meanscomprises a material selected from the group consisting of platinum andchromel.
 12. The apparatus of claim 9 wherein said second and said thirdwire means each comprise a material selected from the group consistingof nickel and constantan.
 13. An apparatus for measuring the oxygencontent of a gas comprising;a tube of solid electrolyte material capableof oxygen ion conductivity, said tube having a closed end incommunication with said gas and an open end in communication withatmospheric air, a first platinum electrode disposed on the insidesurface of said tube, a second platinum electrode disposed on theoutside surface of said tube; a heater disposed within said tube forheating said tube; indicating means for indicating the oxygen content ofsaid gas; first and second electrical wire means in series extendingbetween said first electrode and said measuring means; said first andsecond wire means comprising dissimilar metals, respectively, and joinedto each other at a position removed from said first electrode to form afirst thermocouple junction; said first wire means comprising platinumand connected to said first electrode; said second wire means comprisingnickel and connected to said indicating means; third wire meansextending between said second electrode and said indicating means; andsaid third wire means comprising nickel and joined to said secondelectrode to form a second thermocouple junction.
 14. The apparatus ofclaim 13 wherein said gas is flue gas.
 15. The apparatus of claim 13wherein said electrolyte is stabilized zirconia.
 16. A method formeasuring the oxygen content of a gas comprising;passing said gas to theoutside of a tube of solid electrolyte material capable of oxygen ionconductivity, said tube having a closed end in communication with saidgas and an open end in communication with atmospheric air, said tubehaving an outside electrode and an inside electrode; heating said tubeto maintain a control temperature at said tube; the oxygen in said gasgenerating an electrical signal at said tube and passing said signalthrough said electrodes to an oxygen indicating means for indicating theoxygen content of said gas; measuring the ambient temperature in thevicinity of said tube; and automatically compensating for any offset inindication of oxygen content due to a difference between said ambienttemperature and said control temperature.
 17. The method of claim 16wherein said measurement of ambient temperature is a thermocouplemeasurement.
 18. The method for measuring the oxygen content of a gascomprising;passing said gas to the outside of a tube of solidelectrolyte material capable of oxygen ion conductivity, said tubehaving a closed end in communication with said gas and an open end incommunication with atmospheric air; said tube having an outsideelectrode and an inside electrode; heating said tube to maintain acontrol temperature at said tube; the oxygen in said gas generating afirst electrical signal at said tube and passing said first electricalsignal through said electrodes to an oxygen indicating means forindicating the oxygen content of said gas; measuring by thermocouple theambient temperature in the vicinity of said tube to produce a secondelectrical signal based on the difference between said controltemperature and said ambient temperature; passing said second electricalsignal through said electrodes to said oxygen indicating means; and saidfirst electrical signal and said second electrical signal opposing eachother thereby automatically compensating for any offset in theindication of oxygen content due to a difference between said controltemperature and said ambient temperature.